1. Field of the Invention
The present invention relates to a fuel cell system and a method of driving the same, and more particularly, to a polymer electrolyte fuel cell system and a driving method thereof.
2. Description of the Related Art
A fuel cell system is a power system that directly converts the energy of a chemical reaction of hydrogen in a hydrocarbon-based material such as methanol, ethanol, and natural gas with oxygen into electrical energy.
The fuel cells are, depending upon the kinds of electrolytes to be used, classified into phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, polymer electrolyte fuel cells, or alkaline fuel cells. The respective fuel cells are operated based on fundamentally the same principle, but are differentiated in the kind of fuels to be used, the operation temperature, the catalyst, and the electrolyte.
Among the fuel cells, a polymer electrolyte membrane fuel cell (PEMFC) has been recently developed with excellent power generation capacity, lower operation temperature, and rapid starting and short response time characteristics, compared with other fuel cells. The PEMFC may be widely used for mobile power for cars, distributed power for household and public buildings, and as a small power supply for electronic appliances.
The PEMFC has a basic system structure with a stack, a reformer, a fuel tank, and a fuel pump. The stack forms a main body of the fuel cell system, and the fuel pump supplies a fuel from the fuel tank to the reformer. The reformer reforms the fuel to generate hydrogen gas, and supplies the generated hydrogen gas to the stack. With the above-structured PEMFC, the fuel stored in the fuel tank is supplied to the reformer by way of the pumping pressure of the fuel pump, and the reformer reforms the fuel to generate hydrogen gas. The stack makes the hydrogen gas electrochemically react with oxygen, thereby producing electrical energy.
With the fuel cell system, the stack substantially generates electricity, and has a laminated structure with several to several tens of unit cells based on a membrane electrode assembly (MEA) and a separator (also called a bipolar plate). The membrane electrode assembly has a structure where an anode (also called “fuel electrode” or “oxidation electrode”) and a cathode (also called “air electrode” or “reduction electrode”) are sandwiched by interposing a polymer electrolyte film containing hydrogen ion conductive polymers therebetween. The bipolar plate simultaneously conducts the roles of dividing the membrane/electrode structure and supplying hydrogen and oxygen required for the fuel cell reaction to the anode and the cathode thereof, and as a conductor for connecting the anode and the cathode of the respective membrane/electrode structures in series. The fuel containing hydrogen such as an aqueous methanol solution or reforming gas is supplied to the anode through the bipolar plate, while the oxidant containing oxygen such as air is supplied to the cathode. In this process, the oxidation reaction of hydrogen gas occurs in the anode while the reduction reaction of oxygen occurs in the cathode, thereby producing electrons for electricity, and incidental heat and moisture.
The anode and the cathode of the fuel cell contain catalysts for facilitating the oxidation of the fuel and the reduction of oxygen. With the PEMFC, the catalysts for the cathode and the anode are made by diffusing platinum particles into a carrier of amorphous carbon.
Meanwhile, when the power generation of the fuel cell system is stopped, switching valves on the anode and cathode sides are closed to thereby stop the supply of hydrogen and oxygen. However, during the process of closing the switching valves, the hydrogen gas and the oxygen gas are partially injected into the regions of the anode and the cathode located close to the switching valves. The injected hydrogen gas flows to the cathode through the electrolyte film, and reacts with oxygen in the cathode. Consequently, energy level of the cathode is increased. When the energy level of the cathode is increased, platinum is oxidized so that it deteriorates in terms of catalyst activity, and is dissolved so that the catalyst area is reduced. Particularly when the heightened cathode level is maintained for a long period of time, such problems become serious, and the life span of the fuel cell stack shortens. In order to solve this problem, a technique for removing remnants of hydrogen and oxygen by using a nitrogen-based purge unit has been developed to lower the heightened cathode level. However, the usage of such a purge unit is limited to the inside of a laboratory, and requires a long period of time, up to one or more hours, to remove the remnant gas, and a large volume. Furthermore, the nitrogen-based purge unit requires an additional cost for the nitrogen usage, and hence it is practically difficult to use such a unit in a fuel cell system.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.